Update from https://crrev.com/333737

build/toolchain/win/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
   # Path to the directory containing the VC binaries for the right
   # combination of host and target architectures. Currently only the
   # 64-bit host toolchain is supported, with either 32-bit or 64-bit targets.
   # If vc_bin_dir is not specified on the command line (and it normally
   # isn't), we will dynamically determine the right value to use at runtime.
@@ skipping 21 matching lines @@
                                windows_sdk_path,
                                visual_studio_runtime_dirs,
                                current_cpu,
                              ],
                              "scope")
 
 if (vc_bin_dir == "") {
   vc_bin_dir = toolchain_data.vc_bin_dir
 }
 
+if (use_goma) {
+  goma_prefix = "$goma_dir/gomacc.exe "
+} else {
+  goma_prefix = ""
+}
+
 # This value will be inherited in the toolchain below.
 concurrent_links = exec_script("../get_concurrent_links.py", [], "value")
 
 # Parameters:
 #  current_cpu: current_cpu to pass as a build arg
 #  environment: File name of environment file.
 template("msvc_toolchain") {
   if (defined(invoker.concurrent_links)) {
     concurrent_links = invoker.concurrent_links
   }
 
   env = invoker.environment
 
   if (is_debug) {
     configuration = "Debug"
   } else {
     configuration = "Release"
   }
   exec_script("../../vs_toolchain.py",
               [
                 "copy_dlls",
                 rebase_path(root_build_dir),
                 configuration,
                 invoker.current_cpu,
               ])
 
-  if (use_goma) {
-    goma_prefix = "$goma_dir/gomacc.exe "
-  } else {
-    goma_prefix = ""
-  }
-
-  cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
+  cl = invoker.cl
 
   toolchain(target_name) {
     # Make these apply to all tools below.
     lib_switch = ""
     lib_dir_switch = "/LIBPATH:"
 
     tool("cc") {
       rspfile = "{{output}}.rsp"
       pdbname = "{{target_out_dir}}/{{target_output_name}}_c.pdb"
       command = "ninja -t msvc -e $env -- $cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
@@ skipping 109 matching lines @@
     tool("copy") {
       command =
           "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
       description = "COPY {{source}} {{output}}"
     }
 
     # When invoking this toolchain not as the default one, these args will be
     # passed to the build. They are ignored when this is the default toolchain.
     toolchain_args() {
       current_cpu = invoker.current_cpu
+      if (defined(invoker.is_clang)) {
+        is_clang = invoker.is_clang
+      }
     }
   }
 }
 
 # TODO(dpranke): Declare both toolchains all of the time when we
 # get it sorted out how we want to support them both in a single build.
 # Right now only one of these can be enabled at a time because the
 # runtime libraries get copied to root_build_dir and would collide.
 if (current_cpu == "x86") {
-  msvc_toolchain("32") {
+  msvc_toolchain("x86") {
     environment = "environment.x86"
-
     current_cpu = "x86"
+    cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
+    is_clang = false
+  }
+  msvc_toolchain("clang_x86") {
+    environment = "environment.x86"
+    current_cpu = "x86"
+    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
+                         root_build_dir)
+    cl = "${goma_prefix}$prefix/clang-cl.exe"
+    is_clang = true
   }
 }
 
 if (current_cpu == "x64") {
-  msvc_toolchain("64") {
+  msvc_toolchain("x64") {
     environment = "environment.x64"
-
     current_cpu = "x64"
+    cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
+    is_clang = false
+  }
+  msvc_toolchain("clang_x64") {
+    environment = "environment.x64"
+    current_cpu = "x64"
+    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
+                         root_build_dir)
+    cl = "${goma_prefix}$prefix/clang-cl.exe"
+    is_clang = true
   }
 }